This invention relates to selective ionization apparatus and methods, especially for use in mass spectrometry (MS), ion mobility spectrometry (IMS), gas chromatography (GC), and similar analytical technologies.
The term "ionization" is used herein to describe any process of ion formation in which molecules are converted into charged particles, whether by loss or gain of electrons or by loss or gain of ions.
Gas mixtures are most commonly analyzed at present by GC, MS or IMS. With GC, a special carrier gas and several minutes of processing time are usually required to produce analytical results. With MS, a vacuum of 10.sup.-4 torr or better is indispensable. Operation at atmospheric pressure and at high speed is achieved with MS, but the selectivity and dynamic range of IMS are much poorer than those obtained with either GC or MS.
It is an object of this invention to provide selective ionization apparatus and methods that will greatly improve the analytical selectivities especially of IMS devices and also of GC, MS and related instruments.
The term "analyte" as used herein refers to a substance of interest which is to be detected and whose concentration in a gaseous mixture is to be measured. It is an object of this invention to provide improved means of detecting and measuring very low concentrations of an analyte in air or in other gaseous mixtures in the presence of much higher concentrations of interfering species.
In current IMS technology, a gas sample enters an IMS assembly via a membrane at typically atmospheric pressure and at a selected temperature. The sample mixes with the gas inside a chamber that contains an ion source and an electric field. The ion source, Ni-63, produces a fixed number of ions per unit of time. The chemicals in the chamber become ionized according to the laws of thermodynamics and are separated according to their charge polarity in the electric field.
In IMS instruments used for detection of chemical warfare (CW) agents, a common ionizable chemical, e.g., water vapor or acetone, is added to the gas to form reactant ions. These ions generate analyte ions by charge transfer reactions that take place with CW compounds, which often have the highest affinity for the charge. A small packet of the chemicals ionized in the source is allowed to enter a time-of-flight drift region where an electric field causes the ions in the packet to move. The ionized chemicals travel at different speeds (mobility) because they have slightly different weight, density, and molecular size. The ions are detected at the end of their flight using an electronic high-gain amplifier.
The IMS devices can detect CW substances and vapors of explosives or of illicit drugs in concentrations of 1 to 10 parts per billion by volume (ppbv). However, as the concentration of an analyte is increased, to say 100 to 500 ppbv, all of the reactant ions will have been used and no increase in output signal amplitude will be obtained (the analyte may form dimers and trimers). This leads to IMS systems with a limited dynamic range of measurable analyte concentrations.
The molecular weights of the singly-charged particles are typically between 50 and 500 atomic mass units, the path length is a few centimeters, the electric field is typically 100 to 200 volts/cm, and the resulting times-of-flight of the ionized analytes are typically 10 to 25 milliseconds. As the ion-molecules of a particular ionized species move along the time-of-flight path, they diffuse, and the resulting signal is considerably broader, in time, than the initial packet. The resulting signal has a resolution of between 20 and 50, where resolution is defined as the time width of each signal, measured at half the peak amplitude divided into the time of flight. The level of specificity (the ability to determine that a specific analyte is present because of a time-related output signal) is dependent on the charge affinity, ionization energy of the analyte, its adductive ion stability, its concentration, the concentration of reactant ions, as well as the affinity and concentrations of other compounds that are also trying to attract and capture the limited number of charges that are available. The analyte must have a unique velocity (mobility) in order to be detected at a unique time. These factors restrict the ability of IMS to specifically determine that the ionized species that is detected is a particular analyte.
Thus, the IMS hardware, as it is currently configured, has four important limitations:
1. Non-linear response to concentration: An analyte that produces a detectable IMS response at 5 ppbv can be expected, when exposed to concentrations of 50 ppbv and 100 ppbv, to produce responses that can hardly be distinguished from each other. The difference in minimum detection and signal saturation is ordinarily less than three decades for monomers.
2. Memory Effects: Exposure to a part-per million (ppm) concentration of an analyte can contaminate the IMS sensor, requiring hours or even days for clean-up and restoration of sensitivity.
3. Loss of sensitivity and selectivity due to charge stealing: Because the ionization process is competitive, the charges of the analyte ions can be captured by an interfering chemical that has a higher affinity for the charges. In general, IMS signals for mixtures are very difficult to deconvolute due to the interaction between different species in the source and during flight.
4. Loss of selectivity due to similar mobilities: The output signal will have unresolvable overlapping peaks when different ionized species with similar mobilities are in the same sample. Resolution of a diffusion-broadened peak limits the number of different mobilities that can be clearly isolated in a single detector scan.
In brief, present IMS devices utilize radioactive Ni-63 and an injected intermediate ionizable species to effect ionization in the gaseous samples that are to be analyzed. This favors the species that can be converted into the most stable ions while tending to mask other species. It also results, even for the favored species, in a saturation effect which limits the dynamic range of the concentrations that can be measured by IMS. It is therefore another object of this invention to provide a means of selectively ionizing analytes that yields ions of varying stabilities and over a wider range of analyte concentrations.
A microelectronic field ionizer that may be adaptable to selective ionization for IMS, MS and similar analytical technologies has been disclosed by C.A. Spindt in U.S. Pat. No. 4,926,056, issued on May 15, 1990. Spindt's field ionizer is made by a relatively elaborate procedure and allows a rather restricted flow of gas through its tiny orifices. It is therefore yet another object of this invention to provide a field ionizer that is relatively inexpensive and simple to produce and that permits a higher rate of gas flow per unit area of an ionizer array.
These and other objects of this invention will become apparent from the following description and appended claims.